The below Patent Document 1 discloses a technique for diagnosing the moisture content in a fuel cell of a fuel cell system. In this fuel cell system, the rate of change of a cell voltage when the output current of the fuel cell is changed by a certain rate of change is determined, and the moisture content in the fuel cell is diagnosed in accordance with the difference between the rate of change of the cell voltage and the rate of change of the pre-set reference voltage.